This invention relates to a method of controlling the adsorption of a material or materials from an aqueous solution onto a support and, in particular, to regulating the ingredients contained in the solution to accurately generate a desired coating on a support. More specifically, this invention relates to a method for controlling the amount and spatial distribution of given materials along the length of a support pore.
In recent years, studies have been undertaken in an effort to better understand the adsorption process in order to more accurately generate concentration profiles on a support. The design of concentration profiles for active catalytic materials has received considerable attention in the automotive industry due to the concern surrounding air pollution by engine exhaust emissions. Metal catalysts have been developed which are capable of oxidizing carbon monoxide and hydrocarbons contained in such exhaust gases. Poisons found in these emissions attack the metal catalysts and rapidly deactivate the catalytic coating within a narrow band near the support surface. Accordingly, efforts have been directed toward developing a system wherein a first metal is adsorbed on the support surface to interact with the poisons while a second metal is adsorbed within pores or small crevices contained in the support. This type of construction is highly effective when the process is not diffusion limited.
The incipient wetness technique is commonly used to prepare highly dispersed metallic materials on porous oxide supports. The pores are impregnated with an aqueous solution containing the active catalytic material which is subsequently adsorbed on the pore walls. The catalytic ingredient is typically added in the form of a dissolved salt. Calcination, reduction or other appropriate pretreatment techniques are generally necessary to convert the impregnated crystallite formed by the salt into a catalytically active form. Any of these conversion steps can affect the concentration profile of the catalytic material.
Generally, when a porous support is placed in an aqueous solution containing adsorbable ions, the more active ions which have a higher affinity for the support will concentrate at the entrance to the pores and produce an eggshell-like coating over the support surface. The coating tends to close the pore openings and thus adversely affects the ability of the coating material or a second adsorptive material from penetrating into the pore. The use of additional ingredients as a means of controlling the concentration profile of platinum on an alumina support was first described by Maatman, R. W., Ind. Eng. Chem., 51 (8), 913 (1959). Uniform profiles were obtained by adding acids to the impregnating solution. More recently, further work by Hegedus et al, Preparation of Catalysts II, Elsevier Scientific Publishing Comp., Amsterdam, (1979) described competing ingredients as site blocking agents. This approach allows for mathematical modeling of specific multicomponent processes wherein the adsorption, transport and kinetics are modeled within the pores and an empirical procedure is developed to fit the specific scheme. It should be noted that these prior techniques neglect the complicated solution--support interface chemistry and the methods offer no explanation as to why specific ingredients produce different concentration profiles. The nature of the adsorption process particularly on a porous support is governed by factors beyond simple transport considerations. Such factors as the acid-base equilibrium of the solution, the chemical and crystal structure of added ingredients and the ionic strength determined by the composition of the system must also be considered when describing the process.
In short, most of the information that has been gathered concerning the generation of concentration profiles has been empirical. There is little in the open literature which theoretically explains how solution ingredients produce a given profile. As will be explained in greater detail below, the amount and uniformity of adsorbed materials can be accurately controlled for any given solution by dividing the ingredients into three readily identifiable classes. Each of these affects the process differently. Desired concentration profiles can thus be accurately modeled and the solution adjusted to produce the desired result.